The present invention generally relates to thermoplastic molding compositions and, more particularly, to thermoplastic molding compositions comprising terpolymers of .alpha.-methylstyrene, methyl methacrylate and N-vinyl-2-pyrrolidone which are especially suited for use in injection molding.
Copolymers of .alpha.-methylstyrene and methyl methacrylate, made by known polymerization techniques, have been used to provide resins having relatively high heat distortion properties but which are characterized by relatively poor melt flow characteristics. As a result, processing such as, for example, injection molding, of these copolymer materials requires relatively high temperatures and presents other difficulties making their use in such operations less than fully satisfactory.
Efforts to overcome the foregoing disadvantages have involved copolymerizing minor amounts of alkyl acrylates, long chain alkyl methacrylates, vinyl stearate or vinyl propionate in molding compositions containing these copolymers to improve the flow rate. These comonomers, however, greatly reduce the heat distortion temperature of these .alpha.-methylstyrene and methyl methacrylate copolymers.
In accordance with an important aspect of the present invention, it has been found that terpolymers having a weight ratio from about 20 to 50 parts of .alpha.-methylstyrene, about 79 to 40 parts methyl methacrylate, and about 1 to 10 parts N-vinyl-2-pyrrolidone provide molding compositions which overcome the disadvantages of the prior art .alpha.-methylstyrene and methyl methacrylate copolymer molding compositions. In particular, the terpolymer molding compositions of the present invention have high heat resistance (i.e. they possess high heat deflection temperatures) and are also characterized by improved melt flow rate and transparency. As such, the improved terpolymer thermoplastic molding compositions of the present invention find particularly advantageous utility as injection molding compositions.
It is, therefore, an object of the present invention to provide a terpolymer composition characterized by improved transparency, melt flow and heat deflection properties.
Another object of the present invention is to provide an improved thermoplastic molding composition which is especially suited for injection molding operations.
Still another object of the present invention is to provide an improved molding composition comprising a terpolymer of .alpha.-methylstyrene, methyl methacrylate and N-vinyl-2-pyrrolidone which composition is characterized by high heat resistance as well as improved melt flow rates and transparency when compared to prior art thermoplastic molding compositions based on copolymers of .alpha.-methylstyrene and methyl methacrylate.
These and other objects of the present invention will be apparent from the following further detailed description thereof.
The terpolymer compositions of the present invention can be produced by employing known polymerization processes with monomer mixtures of .alpha.-methylstyrene, methyl methacrylate and N-vinyl-2-pyrrolidone. These monomer mixtures are combined in amounts which, when reacted at polymerization conditions, produce a terpolymer having a weight ratio of from about 20 to 50 parts .alpha.-methylstyrene, about 79 to 40 parts methyl methacrylate, and about 1 to 10 parts N-vinyl-2-pyrrolidone. Preferably, the terpolymer compositions of the present invention have from 20 to 30 parts .alpha.-methylstyrene, 73 to 67 parts methyl methacrylate and 3 to 6 parts N-vinyl-2-pyrrolidone. Particularly preferred terpolymer compositions of the present invention have about 25 parts .alpha.-methylstyrene, 70 parts methyl methacrylate and 5 parts N-vinyl-2-pyrrolidone.
In accordance with an important aspect of the present invention, it has been found that the addition of N-vinyl-2-pyrrolidone to .alpha.-methylstyrene/methyl methacrylate copolymers provides a resultant terpolymer reaction product characterized by a substantially increased melt flow rate and a high heat distortion temperature enabling such terpolymer compositions to be highly suited for molding at relatively low temperatures. Accordingly, an important aspect of the present invention concerns the discovery that the addition of controlled amounts of N-vinyl-2-pyrrolidone as a monomer reactant with .alpha.-methylstyrene and methyl methacrylate results in the production of thermoplastic molding compositions having excellent clarity and improved melt flow rates together with high heat distortion temperatures when compared to conventional .alpha.-methylstyrene/methyl methacrylate copolymer compositions. For example, a particularly preferred embodiment of the present invention is represented by a terpolymer containing 25 parts .alpha.-methylstyrene, 70 parts methyl methacrylate, and 5 parts N-vinyl-2-pyrrolidone which terpolymer is characterized by substantially improved melt flow rates (i.e. melt flows of 1.50 and 8.37 for 1.2 kg. and 3.8 kg. loads at 230.degree. C., respectively, determined and measured in accordance with ASTM Method D-1238) together with a substantially increased heat distortion temperature (i.e. a heat distortion temperature of 275.degree. F. under 264 psi, measured in accordance with ASTM D-648 procedure).